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298
CHEMICAL ENGINEERING
natural clays or the slips prepared from them may have a range of diameter of particle not much outside of 0.005 mm. and from that down to 0.001.1 According to Ries2 the classification of detrital material according to size would be beginning with a fineness represented by "gravel:"
Conventional name	Diameter		
	Inches		Millimeters
Gravel               ......................	K2 I/ Hoo K,5 M,0	to Ms to Ho to Hoo tO ^50 to Hoo tO K)500 00 tO HjOOO DO tO HsjOOO	2.0     to 1.0 1.0     to 0.5 0..5     to 0.25 0.25   to 0.1 0.1     to 0.05 0.05   to 0.01 0.10   to 0.005 0.005 to 0.001
Coarse sand .........................			
Medium sand                   .............			
Fine sand                     ................			
Very fine sand ......................			
Silt ................................			
Fine silt                               ..............			
Clav                                  ..............			
			
lead-pencil manufacturing requires pressures
dy been cited which required 2 tons per square
cent.    When one compares pressures such as
ire which can be obtained in vacuum nitration
VJ. *«„„ „««,« **.. _. x________A---- *c pressure at sea level) the advantage of pressure
filtering in certain filtering problems is at once apparent.    In the vacuum filter low pressure is offset to some extent by making a thin cake.
FIG. 19.—Square-leaf filter press.
In plate and plate and frame presses certain filtering media can be used which cannot be employed on vacuum filters. In clarifying certain vegetable oils, filter paper must be used and quite apparently such a medium cannot be used on a rotary vacuum filter.
i G. F. MERILL, "The Non-metallic Minerals," p. 233. 8 Op. cit.t p. 116.